Inland Fisheries Ireland has launched an investigation into a "serious incident" near Kinsale in Co Cork, in which at least 2,000 fish died.
The fish kill took place over a near-5km stretch of water on the Brownsmills stream, flowing into the estuary at Kinsale.
Among the species identified were brown trout and eel.
Fish and water samples have been taken for analysis, and Cork County Council's Environmental Department are assisting with the investigation.
IFI says it was first made aware of the incident yesterday after a member of the public noticed dead fish in the stream.
The agency says its investigation is at an early stage, and so it is not yet able to provide a cause for what happened.
